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SHOETEE AETICLES AND EEPOBTS 

THE SIMULTANEOUS MODIFICATION OF DISTINCT 
MENDELIAN FACTORS 

In another paper 1 on the inheritance of a recurring somatic 
variation in variegated ears of maize, it was shown that the 
amount of red color developed in the pericarp of variegated 
seeds bears a definite relation to the development of color in the 
progeny of such seeds. The relation is such that the more color 
there is in the pericarp of the seeds planted the more likely are 
they to produce plants with wholly self-red ears and correspond- 
ingly the less likely to produce plants with variegated ears. 
Self-red ears thus produced behave just as if they were hybrids 
between self-red and variegated races or self -red and white races, 
the behavior in . any given case depending upon whether the 
parent variegated ears were homozygous or heterozygous for 
variegated pericarp and whether they were self-pollinated oi" 
crossed with white. 

To interpret these facts I have suggested that perhaps (1) a 
Mendelian factor for variegation, V, is changed to a self-color 
factor, S, in a somatic cell, (2) that all pericarp cells directly 
descended from this modified cell develop red color, and (3) 
that of the gametes arising from modified cells one half carry 
the S factor and one half the V factor. 2 "Whether it ever hap- 
pens that more than one half of such gametes carry S is un- 
known, but it is certain that a considerable part of them carry V. 
This is shown by the fact that self-red seeds from a variegated 
ear that has been cross-pollinated by white-eared maize produce 
a considerable percentage of variegated-eared plants. Evidently 
in such cases the duplex condition of the factors is changed to 
the simplex condition by the change of one V factor to an S 
factor, so that the zygotic formula VV becomes VS. 

Now it often happens that a considerable patch of self-red 
grains occurs on an otherwise variegated ear. The cob imme- 
diately beneath such a patch is sometimes variegated, just like 

1 Not yet in print. 

3 This hypothesis was noted in my discussion of the possible origin of 
mutations in somatic cells. American Naturalist, 47: 375-377, 1913. 

633 



634 THE AMERICAN NATURALIST [Vol. XL VII 

that beneath the variegated grains of the same ear, and some- 
times self-red, the red cob spot corresponding exactly with the 
patch of red grains. Yet the wholly red ears arising from such 
self-red seeds invariably have wholly red cobs without respect 
to whether the parent seeds were from a red or variegated cob 
spot. It seems possible that in some cases the change from V 
to S occurs earlier in the life of the plant than in other cases. 
In some plants the change may, it seems possible, occur soon 
after the cob is laid down, in which case all the cells of the 
glumes as well as of the pericarp over a considerable area will 
be red. In other plants it appears that the change from V to 8 
occurs independently in the rudiments of several grains, but not 
until after the glumes associated with them have been laid down. 
But in either case, it must be remembered, the red ears produced 
from such red seeds always have wholly red cobs as well as 
wholly red grains and cob and pericarp colors are coupled in all 
later generations. Evidently, whatever is responsible for the 
change from variegation to self-color always affects both cob 
and pericarp colors. 

This would occasion no surprise if it were known that red cob 
color and red pericarp color are due to identical factors. But 
I have presented, in another place, 3 evidence that cob and peri- 
carp colors are dependent upon distinct genetic factors which 
are either coupled or allelomorphic in inheritance. Even if it 
should be shown that the red color of the cob is due to identically 
the same pigment as the red color of the pericarp, it must never- 
theless be assumed that there are distinct genetic factors that 
influence the distribution of this pigment. The factor S c that 
has to do with the determination of self-pattern of cob color can 
hardly be the same as the factor S p that has to do with the same 
pattern in the pericarp, for, if it were the same, a cross of a 
strain having variegated cob and variegated pericarp with a 
strain having self-red cob and colorless pericarp should produce 
progeny self-red in both cob and pericarp, whereas such a cross 
actually produces ears with self-red cobs and variegated peri- 
carp. We are practically driven, therefore, to the conclusion 
that there must be distinct factors for self-color of the cob and 
self -color of the pericarp, 8 C and S p , respectively. It seems 
reasonable then to suppose that the same is true of the variega- 
tion pattern and that there are both V c and V p for variegated 
cob and variegated pericarp, respectively. 
8 Ann. Rpt. Nebr. Agr. Expt. Sta., 24: S9-90, 1911. 



No. 562] SHORTER ARTICLES AND REPORTS 635 

If this is true, we are confronted with the problem of explain- 
ing the apparently universal occurrence of self-red cobs in con- 
nection with self-red ears arising in F t from variegated-eared 
parents. Why, in short, should V c and V P , if they are really 
distinct, always change together to S and 8 P , whenever either 
one changes? This seems the more unaccountable when consid- 
ered in connection with the fact that the change often, or per- 
haps always, affects only one of the two like (duplex) factors 
of a homozygous somatic cell, so that V C V P ■ V C V P becomes 
S C S P ■ V C V P . 

In my former paper {loc. cit.) I accounted for perfect 
coupling of cob and pericarp factors in certain crosses by the 
assumption that the two factors were located in the same chro- 
mosome, and explained perfect allelomorphism of the same fac- 
tors in other crosses by the assumption that the two f actors were 
located in homologous chromosomes. This was on the further 
assumption that homologous chromosomes separate at the reduc- 
tion division exactly at the plane of their union in synapsis. 
If in place of this last assumption, however, we accept Morgan's 1 
suggestion, based upon cytological evidence presented by Jans- 
sens, that homologous chromosomes may become spirally twisted 
together in synapsis and that the plane of separation may not 
always coincide exactly with the plane of union, we must also 
accept his further suggestion that the linear position of factors 
within a chromosome has much to do with the degree of coupling 
and allelomorphism, "linkage." To me Morgan's hypothesis 
seems the most reasonable interpretation of the facts of partial 
coupling and "repulsion," and it also affords a satisfactory ex- 
planation of perfect coupling and allelomorphism. 

In accordance with Morgan's hypothesis, we must suppose, 
not only that the factors V c and V P are located in the same chro- 
mosome as I had done before, but in addition that they are situ- 
ated very close together in this chromosome, since their linkage 
seems to be perfect. Similarly we must suppose, not only that 
V p and Sp are in homologous chromosomes, as I had previously 
done, but that they are in almost exactly homologous positions 
in these chromosomes, since their allelomorphism appears to be 
perfect. This second supposition follows of course as a corol- 
lary of the first one if S is produced through a modification of V. 

Now we might suppose further that the two factors, V p and 

* Science, N. S., 34: 384, 1911. 



636 THE AMERICAN NATURALIST [Vol. XL VII 

V are located side by side in the same chromosomes not only at 
the time of the reduction division but also in all nuclear divi- 
sions and even perhaps that they remain in fairly close prox- 
imity in the more diffused chromatin of the resting nucleus. 
Then if homologous chromosomes or their chromatin masses are 
not closely associated in somatic cells, it would seem possible that 
whatever causes the change of a V p factor into an S p factor 
might at the same time affect the V c factor of the same chromo- 
some changing it into an S c factor, while the V p and V factors 
of the homologous chromosome remain unchanged. 

It is of course recognized that a rather formidable number of 
hypotheses, with subsidiary assumptions, have been marshalled 
here to account for what may be very simple phenomena, but, 
if they do not do too great violence to the known facts of cytol- 
ogy, we are justifiable in accepting them tentatively as an 
attempt at a consistent interpretation of what otherwise seem 
inconsistent genetic facts. 

R. A. Emerson 

University of Nebraska 



THE FOURTH INTERNATIONAL GENETICS 
CONFERENCE 1 

In a subject developing so rapidly as that of genetics, the 
delay of one and one half years in the publication of the results 
of an investigation is a serious matter. It is therefore to be 
regretted that the publication of the proceedings of the Fourth 
International Conference on Genetics has followed the common 
fault of international congresses in this respect. In many cases 
results which were new at the time of the conference have been 
anticipated by other work. In other cases the results of later 
experiments have no doubt served to modify opinions expressed 
at the conference. A portion of this delay is inherent in the 
nature of an international meeting. However, it is hoped that 
for the coming conference, steps will be taken to insure the more 
rapid publication of the. proceedings. 

The present volume of 570 pages consists of two parts. Part 

I (pages 1 to 79) contains the matter of historical interest relat- 

i ' ' Comptes Rendns et Rapports de IV e Conference Internationale de 
G-engtique. ' ' Edites par Ph. de Vilmorin. x + 571 pp. Masson et . Cie, 
Paris. 1913.